'First Day' in the Bible

And it came to pass in the fortieth year, in the eleventh month, on the first day of the month, that Moses spoke to the children of Israel, according to all that the LORD had given him in commandment to them;

And there shall be no leavened bread seen with thee in all thy borders seven days; neither shall there any thing of the flesh, which thou didst sacrifice the first day at evening, remain all night until the morning.